This invention relates to the hydrofining of hydrocarbon streams and more particularly to the hydrofining of such streams employing multistage catalytic hydrofining for the reduction of sulfur contaminants in such streams.
The hydrotreating of hydrocarbon streams over catalyst systems employing Group VI and Group VIII metals is well known in the chemical processing industry. Such hydrotreating of petroleum feedstocks, which can be derived from crude oil distillation units, cracking units, and other units involved in oil refining operations, typically involve a so-called hydrofining operation in which a hydrocarbon feedstock containing unwanted sulfur and nitrogen components as well as aromatic components are supplied along with hydrogen in order to effect desired contaminant reactions in the feedstock. In such operations, it is desired to effect hydrotreating and hydrodesulfurization reactions with the result that the sulfur and nitrogen content of the feedstock is substantially reduced and hydrogenation of various unsaturated hydrocarbon components in the feedstock e.g., the hydrogenation of aromatics, takes place. Typically the catalyst systems employed in such processes involve metals found in Group VI and Group VIII of the Periodic Table of the Elements. For example, U.S. Pat. No. 5,198,099 To Trachte discloses a so-called three-stage process for the refining of distillate streams. The first stage involves hydrotreating the feedstock at a temperature ranging from about 200xc2x0-400xc2x0 C. and a hydrogen gas rate ranging from 4000-6000 SCF/barrel over a catalyst comprised of at least one Group VIII metal and a Group VI metal on an inorganic oxide support. Specifically disclosed among the Group VIII metals are cobalt and nickel. The preferred Group VI metal is molybdenum. Preferred supports are said to be alumina and silica-alumina with the former most preferred. The effluent from the first hydrotreating stage is supplied to a second hydrotreating stage conducted under milder conditions about 190xc2x0 C. to 360xc2x0 C. to remove additional sulfur and nitrogen while keeping cracking at a minimum. The heavier fractions output from the second stage is supplied to a third hydrocracking stage operated at a temperature of 200-370xc2x0 C. The catalyst employed in the second and third stages is described as a conventional hydrocracking catalyst, typically a Group VIII catalyst on a zeolite cracking base.
U.S. Pat. No. 5,447,621 to Hunter discloses a refining process described as an integrated process involving a hydrocracking stage, a product fractionation stage, and a hydrotreating stage. Catalysts disclosed for use in the hydrotreating reactions, which are described as including aromatic hydrogenation, catalytic dewaxing, hydroprocessing hydrodenitrogenation, and hydrodesulfurization, include cobalt molybdenum and nickel molybdenum base metals or a noble metal catalyst. U.S. Pat. No. 6,123,830 to Gupta et al also discloses an integrated refining process which involves a hydroprocessing procedure which incorporates at least two hydrotreating steps and at least two catalytic cracking reactions, preferably performed in series. The hydroprocessing reactions typically are carried out over trickle bed reactors involving a concurrent downflow of liquid and gas phases over fixed catalyst beds. The catalyst employed are said to be suitable for aromatic saturation, desulfurization, or denitrogenation or combinations thereof and include at least one Group VIII metal and a Group VI metal on an inorganic refractory support. A preferred support is said to be alumina or alumina-silica. The silica-alumna supports include crystalline alumino-silicates such as zeolites. The Group VIII metals include cobalt, nickel, and iron, and the preferred Group VI metal is molybdenum.
Yet another process for the hydrotreating of process streams for the removal of heteroatoms incorporating sulfur from such streams is disclosed in U.S. Pat. No. 5,198,100 to Aldridge et al. The various feedstocks disclosed in Aldridge are light, middle, and heavy petroleum distillates, petroleum residual feeds, and coal-derived liquids, shale oil, and heavy oils from tar sands. The catalysts employed in the Aldridge et al process incorporate salts or complexes of at least one Group VIII metal, cobalt and/or nickel, with at least one Group VI metal heteropolyacid, preferably molybdenum or tungsten on an inorganic oxide support, preferable alumina. Supports for the Group VI and Group VIII supports can also include silica-alumina. After formulating the supported catalyst, the support can be impregnated with appropriate metal salts followed by a Group VIII heteropolyacid deposit on the support. The catalyst is then dried and prior to use is sulfided with a sulfur-containing distillate. This is followed by the hydrotreating procedure. The sulfiding feed can take the form of a 7.4 % dimethylsulfide and 92.6% of petroleum distillate.
In accordance with the present invention there is provided a multi-stage hydrofining process incorporating separate stages of cobalt molybdenum catalysts. In carrying out the invention, a sulfur- and hydrocarbon-containing processing stream is supplied to a multi stage hydrotreating reactor. The reactor is operated under conditions effective to accomplish the hydrofining reactions in the presence of hydrogen which may be supplied concurrently or countercurrently with the hydrocarbon-containing processing stream. Within the reactor, the processing stream is passed initially into contact with an initial catalyst stage comprising a cobalt molybdenum desulfurization catalyst. This catalyst is present in a minor amount preferably an amount within the rage of 10-40% of the total composite amount of catalysts within the reactor. Thereafter the processing stream is passed through a subsequent catalyst stage comprising a major amount of a cobalt molybdenum hydrocracking catalyst. Preferably, the cobalt molybdenum catalyst in the subsequent catalyst stage comprises from about 60-90% of the composite of the cobalt molybdenum catalysts. The effluent stream having a reduced sulfur content is then withdrawn from the hydrotreating reactor where it can be made available for further processing.
Preferably, the initial and subsequent catalyst stages within the reactor are separated by an intervening sector which occupies a relatively small section within the reactor, thus providing a travel path in the reactor which is less than the travel path of the initial catalyst stage. This intervening sector contains an inert particulate refractory material of low thermal conductivity. In a particularly preferred embodiment of the invention the intervening particulate material comprises silica particles which are generally spheroidal in shape.
In the preferred embodiment of the invention, the cobalt molybdenum catalyst employed in the initial catalyst stage is supported on an alumina support and the cobalt molybdenum catalyst in the subsequent catalyst stage is supported on a silica-alumina support having a greater acidity than the alumina support for the initial cobalt molybdenum catalyst. Preferably, the cobalt molybdenum catalyst in the initial stage comprises about 10-30 wt. %, more preferably 10-20 wt. %, of the total composite catalysts, and the catalyst in the subsequent stage, about 70-90 wt.%, more preferably 80-90 wt. %, of the total composite amount of the catalysts within the reactor.